If you want to alert your colleague or client about what happened in the task, set up the notification at the bottom of the comment editor. Click on icon for adding a follower (Add/Remove follovers…) and select other people to get notifications.
Animation: assigning users to track a task.If you are looking for a way to remind users only once, then look at @mention. You can write directly to the comment.
TIP: Do not be afraid to remove the user tracking
In the same way, you will be stripped of your colleague and no more warnings will come to him.
This is usually the case for long discussions where too many people are alerted. Always think about who the new information will help finish the task.